This invention relates to the optical measurement of the alignment error of the three degrees of freedom of a plurality of coordinate reference frames with the 3.degree. of freedom of a relatively absolute reference frame. In optical alignment and measurement systems, it is relatively simple to provide a system to perform simple alignment measurements, where only 1.degree. of freedom or a single object's position is of concern. As more alignment information is elicited and the position of more objects are measured, the systems become increasingly complex and the number of components of the systems multiply. This increasing complexity and proliferation of elements introduces a greater possibility for error and makes the measurement data obtained and consequent realignment suspect.
One example of a complex system is the launching system of a missile-carrying submarine The submarine-missiles are arranged in two longitudinal rows along either side of the vessel, each row containing, for example, eight missiles. To provide the missiles with a homing capability, a directional target-seeking system including a guidance system is provided aboard each missile. At the instant of launch each missile's guidance system must be provided with precise information as to its present bearing, including all three degrees of freedom, if the missile is to find its target. This information has been provided in the past by generating two of the three degrees of freedom by reference to gravity and obtaining the third from the ship's inertial navigation system by an optical alignment system. An example of such a system is found in U.S. Pat. No. 3,709,608.
For numerous reasons, it has been attempted to eliminate the reference to gravity to obtain 2.degree. of freedom of the missile bearing and instead to determine all 3.degree. of freedom optically by comparison with the ship's gyro.
Prior art optical alignment and measurement devices have not been capable of accurately measuring the difference of alignment between all 3.degree. of freedom of a plurality of coordinate reference frames and the three degrees of freedom of a relatively absolute reference frame by means of a simple optical alignment system.
Furthermore, none of the known prior art devices provide a means of measuring, and thus enabling compensating for, errors of optical alignment which may be inherent in these systems.
Additional prior art optical alignment means are known which employ light beams having off-axis angles or multiple beams to determine the three means are not practical. For example, in the missile launching system described above, the missile walls have only one window through which the position of the internally-located guidance system is determined. For this system, it is necessary to employ a single light beam of relatively narrow angular extent. Thus, off-axis light-beam means or multiple-beam means would not be satisfactory for the optical alignment of the components of certain systems.